The invention relates to a receiver for a packet transmission system comprising receiving means for receiving a signal at a symbol frequency, oversampling means for generating oversamples from the received signal at a frequency which is a multiple of the symbol frequency, and a device for searching the optimal sampling instant for estimating, on the basis of the generated oversamples, a delay corresponding to the received signal.
The invention also relates to a packet transmission system comprising such a receiver.
The invention further relates to a method of estimating an optimal sampling instant at the receiver end for a packet transmission system, the method comprising the steps of
receiving a signal referred to as received signal,
oversampling for generating oversamples from the received signal,
searching an optimal sampling instant for estimating, on the basis of the generated oversamples, a delay corresponding to the received signal.
The invention finds important applications in the field of satellite or cable transmissions and notably in return path transmissions in which a plurality of terminals is capable of transmitting data packets to a head station in accordance with a time-division multiplex mechanism.
The article entitled xe2x80x9cDigital Communication Receivers Synchronization Channel Estimation and Signal Processingxe2x80x9d by H. Meyr, M Moeneclay and S. Fechtel published in the Wiley Series in Telecommunications and Signal Processing, pp. 285 to 289, describes a technique for computing the optimal sampling instant. In the majority of time-division transmission systems it is necessary to effect a oversampling at the receiver end so as to retrieve the sampling instant used at the transmitter end. The technique described in the quoted article recommends computation of the average energies of each sample for selecting the optimal sample having the maximum average energy. This technique is costly as regards the number of computations because it necessitates computation of the average energy of all the received samples representing a symbol transmitted before deciding which is the optimal sample.
It is a particular object of the invention to provide the possibility of very rapidly recovering the optimal sampling instant by considerably limiting the number of computations and thus the operating costs. To this end, the receiver as described in the opening paragraph is characterized in that said receiving means comprise a low-pass filter and centering means for centering the low-pass filter as a function of said delay.
In accordance with an important characteristic feature of the invention, the device for searching the optimal sampling instant comprises a computing member for determining the maximum of the average amplitudes of the oversamples, and an interpolator for computing a polynomial through at least three of said average amplitudes, among which the maximum amplitude, and for deriving said delay estimation therefrom.
The invention has the advantage that it provides the possibility of very rapidly recovering the optimal sampling instant with a small number of computations so that it is possible to carry out this operation for every new packet received from any one of the terminals of the system.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.